Removal of fluoride from chloride or bromide melts

ABSTRACT

A method is provided for removing fluoride ion in a molten chloride or bromide melt which comprises passing sufficient gaseous BC13 through the melt to convert the BC13 to BF3 and flushing the BF3 from said melt.

United States Patent [191 Grimes et al.

[ Apr. 23, 1974 REMOVAL OF FLUORIDE FROM CHLORIDE OR BROMIDE MELTS [75] Inventors: Warren R. Grimes; James H.

Shaffer, both of Oak Ridge; Forrest A. Doss, Clinton, all of Tenn.

[73] Assignee: The United States of America as [58] Field of Search 252/3011 R; 23/325, 326, 23/349, 89, 205; 423/293, 179, 499

[56] References Cited UNITED STATES PATENTS 7 3,577,225 5/1971 Shaffer et al. 23/325 2,840,464 6/1958 Wiswall 75/84.] 2,805,130 9/1957 23/205 2,759,790 3/1956 23/326 2,668,750 2/ 1954 Krchma 23/89 OTHER PUBLICATIONS Booth et al., The Fluorination of Boron Trichloride, .1. Am. Chem. Soc. Vol. 65, p. 1836, (1943). QDiAS.

Primary Examiner-Cari D. Quarforth Assistant Examiner-R; L. Tate Attorney, Agent, or Firm-John A. Horan; David S. Zachry; Irving Barrack [5 7] ABSTRACT A method is provided for removing fluoride ion in a molten chloride or bromide melt which comprises passing sufficient gaseous BCI, through the melt to convert the BCI, to BF, and flushing the B1 from said melt.

2 Claims, 2 Drawing Figures A'I'FMTED m 2 3 01-1 r a 06581 SHEET 1 BF 2 19,000 WEIGHT 0F SALTI 1.7? kg BC|3 IN H I0.8miIlimoles/liter H2 FLOW RATE :1.5liiers/min.

FLUORIDE CONCENTRATION IN SALT (ppm by wt) 0\ o o A 50 100 150 200 250 300 REACTION TIME (min.)

INVENTORS.

ATTORNEY.

PATENTEDAPR 23 I974 FLUORIDE REMOVED FROM MELT (equivalents) SHEEI 2 UF 2 Q8 WEIGHT 0F MELT:1.77kq

BCI IN H ZOBmiIIimOIes/Htef H FLOW RATE :LSIiters/min RUN B IN|T|AL F |N MELT BCI3 PASSE D(equiva|enis) INVENTORS.

Warren R. Grimes James H. Shaffer Forrest A. Doss ATTORNEY.

REMOVAL OF FLUORIDE FROM CHLORIDE OR BROMIDE MELTS BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the US. Atomic Energy Commission.

The advantages of this invention are realized by reacting gaseous BC] with molten LiF or mixtures thereof. Under these conditions the BCl is converted to gaseous BF and is visualized as occurring in accordance with the general formula The method is simple, rapid, and effective and does not introduce any extraneous impurities into the system since the boron enters as a gaseous chloride and exits as a gaseous fluoride.

REPRESENTATIVE EMBODIMENT Approximately 1.8 Kg of molten LiCl (650 C.) was loaded into a reaction vessel constructed'from a 16- inch length of 4-inch nickel pipe having one end welded closed and having a top plate provided with a salt sampling port, a thermowell, and a sparge gas line. A known quantity of lithium fluoride was added to the molten LiCl and sparged'with argon for mixing. Boron trichloride admixed with hydrogen was then bubbled into the molten LiCl at 650 C. The hydrogen was used as a carrier gas and to provide a reducing cover atmosphere to avoid oxidation effects. Any other inert cover gas would serve as well, such as helium or argon.

Filtered samples of the molten salt were withdrawn periodically before and during the experimental procedure and analyzed chemically for fluoride content. Samples of the gas influent and effluent streams were periodically collected in water and analyzed for BCl by titration with standard caustic solution.

FIG. 1 shows how the fluoride concentration of several fluoride samples (runs A-D, inclusive) is rapidly diminished to low values with 4-hour reaction periods even at low BCI concentrations of 0.8 millimole/liter of hydrogen in the sparge gas stream. It is seen that initial concentrations of 5,000 to 10,000 ppm were rapidly diminished to low values in slightly more than 4 hours to allow a BC] concentration of 0.8 millimole per liter of H in the sparge gas stream.

Subsequently, experiments were carried out to determine relative reaction velocity constants and equilibrium quotients. As shown by FIG. 2, thereaction of fluoride ion with BCl is essentially quantitative even at low BCl concentrations. Thus, the rate of chemical reaction is controlled by the rate at which BCl is admitted to the system. Values calculated for the equilibrium relation are given by where N is the mole fraction of the entities identified in the subscript which show a continuous increase, approaching a value of 10 as a limit, as the fluoride ion concentration in the melt diminished. Thus we have shown that the adverse quantities of fluoride can be effectively removed from LiCl. In a generic sense,-the invention is useful in any-situation where molten chlorides are contaminated with fluoride ion.

What is claimed is:'

1. A method for removing fluoride ion contained as an impurity in a molten alkali metal chloride which comprises passing sufficient gaseous BCL, through the melt to convert the fluoride ion to BF and flushing the resultant'BF from said melt.

2. The method according to claim 1 wherein the alkali metal molten chloride is lithium chloride. 

2. The method according to claim 1 wherein the alkali metal molten chloride is lithium chloride. 